JPS606119B2 - Composite semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a composite semiconductor device in which a semiconductor laser is combined with other optical functional elements.

Semiconductor lasers have the advantages of being small, lightweight, and highly efficient, and are therefore considered to be a powerful light source for optical communication devices and optical information processing devices.

In particular, distributed feedback semiconductor lasers perform stable single-wavelength oscillation and do not require reflective mirror surfaces, making it possible to monolithically couple them with semiconductor optical functional devices such as semiconductor optical waveguides and semiconductor optical modulators. It is expected to be important as a light source in optical concentrator circuits. However, in conventional distributed feedback semiconductor lasers, the optical absorption in the non-excited part of the semiconductor monolithically coupled to the laser is extremely large, several thousand [1], and a large amount of optical absorption in this part is caused. The loss significantly reduces the practicality of composite semiconductor laser devices or optical integrated circuits that are monolithically coupled with other optical functional elements. Furthermore, regardless of distributed feedback type or not, conventional semiconductor lasers with multilayer structures such as double heterostructures exhibit significant nonlinearity in the relationship between excitation current and output light intensity, and unsteady state during pulsed current excitation. Abnormalities in the characteristics are often observed, such as the amplitude of the relaxation oscillation that appears during oscillation does not decay in the usual simple exponential relationship. Abnormalities in these characteristics, such as abnormalities in response characteristics during direct modulation, pose an obstacle to the practical use of semiconductor lasers. The reason for this is that ``In these semiconductor lasers, optical waveguide in a direction parallel to the interface of the active layer and perpendicular to the direction of light propagation (hereinafter referred to as the horizontal lateral direction) is caused by the spatial distribution of carrier density in this direction. This is caused by the optical gain formed and the spatial distribution of yield rate, and the spatial distribution of carrier density changes as the excitation current increases and decreases, causing instability in which the horizontal transverse mode structure itself changes. The first object of the present invention is to ``develop a distributed feedback type or distributed Bragg reflector type semiconductor laser and other optical functional elements by a method with little optical loss in the non-excited part. An object of the present invention is to provide a monolithically combined composite semiconductor device.

A second object of the present invention is to eliminate the instability of the horizontal mode structure described above and to provide a composite semiconductor device with stable laser characteristics. The composite semiconductor device according to the present invention has a multilayer structure, and includes a semiconductor laser that performs optical feedback by distributed feedback using a diffraction grating provided at the interface of the active layer or a layer adjacent to the active layer, and a semiconductor laser that is monolithically coupled to the semiconductor laser. It is composed of optical functional elements such as a modulator and a semiconductor optical waveguide.

Next, the principle of this invention will be explained in two parts.

A first part of the principles of the invention relates to a method for reducing optical losses in an unexcited region monolithically coupled to a distributed feedback or distributed Bragg mirror semiconductor laser.
Generally, the energy corresponding to the oscillation wavelength of a semiconductor laser is considered to be several to several tens of meV larger than the energy gap in the forbidden band between the conductive band and the valence band. On the other hand, optical absorption in an unexcited semiconductor rapidly increases as the wavelength decreases near the wavelength corresponding to the forbidden band, and increases by several to several tens of meters compared to the forbidden band.
At wavelengths corresponding to large energies (eV), the magnitude of the absorption coefficient reaches several thousand to tens of thousands [c-1]. Therefore, it has been thought that if an unexcited region with the same composition as the active region of the semiconductor laser exists inside the semiconductor laser or outside adjacent to it, the laser light is significantly attenuated in this unexcited region. This idea proves that the forbidden band width of a semiconductor does not depend on the density of free carriers. However, it is theoretically predicted that the effective forbidden band of a semiconductor will shrink as the free carrier density increases, and this has been recently confirmed by various experiments. In gallium diode, the reduction in this forbidden band is, for example, 20 [when the excited carrier density is 1 and 8 [S-3]].
[meV]. Therefore, as the density of excited free carriers increases, the wavelength spectrum of the gain in the excitation region of the semiconductor shifts to a slightly longer wavelength side than the light absorption in the non-excitation region. On the other hand, the wavelength that provides the maximum gain in the gain spectrum also gradually shifts toward longer wavelengths as the excited carrier density increases, but the magnitude of this shift is smaller than the contraction in the forbidden band.
Therefore, the optical absorption in the non-excitation region at the wavelength that gives the maximum value of the gain spectrum is normally extremely large. The gain at this wavelength is sufficiently large, and the absorption at this wavelength in the non-excited region is 1 to 2
It has been confirmed through experiments that it can be reduced to about ×1 pi [skin-1] or less. For a more detailed explanation on this point, please refer to Japanese Patent Application No. 51-51606 (Hakama Kai Sho 52-1343 Patent Publication). Therefore, in a distributed feedback semiconductor laser having a monolithically coupled unexcited waveguide region, the lattice spacing of the diffraction grating provided for distributed feedback is appropriately selected to achieve an excited carrier density of 1 to 2 x 1 to 8 [ If this diffraction grating is set to perform optical feedback efficiently at a wavelength several tens of triangles longer than the wavelength that gives the maximum value of the gain spectrum when the gain spectrum reaches about 3. This distributed feedback semiconductor laser can be made to oscillate in a state where is not very large. Specifically, the above wavelength setting is achieved by, for example, using part of the same wafer or other wafers with the same composition and structure to fabricate several distributed feedback semiconductor lasers with slightly different lattice spacings. This can be done by taking into consideration the excitation current threshold for these oscillations and the amount of attenuation when each output passes through the non-excitation region. The grating spacing and oscillation wavelength set by such a method are hereinafter referred to as the optimum lattice spacing and the optimum oscillation wavelength, respectively. The optimum oscillation wavelength according to the present invention is usually about 50 A or more than the wavelength corresponding to the effective forbidden band in the non-excited state of the active region. The second part of the principles of this invention relates to the changes in effective bandgap and compromise that occur when doping a semiconductor with impurities.

It is well known that by partially adding P-type or N-type impurities to a semiconductor by a method such as diffusion or ion implantation, the density of free carriers in that region can be increased. The effective forbidden band width at that part is 4. Further, although the dielectric constant of a portion doped with impurities generally changes, the magnitude and direction of the change are not constant depending on the type and density of the impurity and the wavelength of interest. However, by selecting an appropriate impurity and concentration, when exciting that part to cause laser oscillation, the power yield at that oscillation wavelength can be increased by 10 degrees in relative terms. For example, , neutral potassium borosilicate or free carrier density at most 1
If zinc is diffused into potassium containing n-type impurities in an amount that reaches a concentration of 1 and 9 [ka-3], the area where the zinc has been diffused will be activated. When laser oscillation is performed as a region, the dielectric constant near the oscillation wavelength increases by 1 to 2 × 10-uniformity in relative value compared to that in a region where zinc diffusion is not performed. Confirmed by experiment. Therefore, using this method, a width of 1
Forming a band-shaped part with a high dielectric constant of about 0 to 20 #m,
If this part is used as an active region and the oscillation is performed,
Since the laser light is guided stably by this portion having a high power yield ratio, various abnormalities in characteristics caused by the instability of the horizontal mode structure described above can be eliminated. Moreover, due to the increase in free carrier density caused by the introduction of the impurities mentioned above, the effective forbidden band is reduced.
Therefore, in a distributed feedback semiconductor laser having a monolithically coupled non-excited region, impurities are introduced into the excited region by a method such as zinc diffusion to reduce the effective forbidden band in this region in advance. By doing so, it is possible to increase the gain in the excitation region and reduce the optical loss in the non-excitation region, compared to the case without diffusion. Next, embodiments of the invention will be described with reference to the drawings.

FIG. 1 is a conceptual diagram of a first embodiment of the present invention. n-
On top of the GaAs substrate 2 are an n-Al-yAs layer 3, which is an active layer. It is manufactured by sequentially epitaxially growing layer 6 and P-GaAs layer 7. Here, y>z>x is the necessary condition and x=0 to 01.
, y=0.3-0.4 z=0.2-0.3 are usually used. P electrodes 10 and 11 are provided on the P-Ga layer 7, and an n-electrode 1 is provided below the n-Ga rock substrate 2. After growing the P-A-Gal-As layer 5, a diffraction grating 14 is provided on a part of its surface using a technique such as photoetching. Because of the optical feedback caused by this diffraction grating 14, the part sandwiched between the front electrode 10 and the back electrode 1 is affected by the current flowing between these electrodes in the direction that the potential of the electrode 10 becomes higher than that of the electrode 1. A distributed feedback semiconductor laser 20 that is excited and oscillates is configured. The portion sandwiched between the electrode 11 and the electrode box constitutes an optical modulator 21 that is driven by a variable voltage applied between these electrodes in a direction in which the potential of the electrode 11 is lower than that of the electrode 1. By setting the grating spacing a of the diffraction grating 14 to the optimum grating spacing, when the output of the distributed feedback semiconductor laser 20 "propagates through the active layer 4 and is guided to the optical modulator 21," It is possible to reduce the optical loss in the excitation region.This also makes it possible to reduce the optical loss in the optical modulator 21 when no voltage is applied to the optical modulator 21, that is, the coupling loss. This means reducing the insertion loss of the modulator, which is practically desirable.Moreover, the energy corresponding to the optimum oscillation wavelength set according to the principles of the present invention is lower than that in the effective forbidden band in the non-excited region of the active layer 4. at most 20-30 [meV]
Since the absorption coefficient within the optical modulator 21 at this wavelength changes extremely sensitively following the voltage fluctuation between the electrodes 1 and 11, the optical modulator 21
The modulation efficiency of is high. FIG. 2 is a conceptual diagram of a second embodiment of the invention. In this figure, the multilayer structures 2 to 7 are manufactured on the same machine as the first embodiment of FIG. 8 is a Si02 film which is an electrical non-conductor.

A window having the position and size of the stripe 13 shown by the dotted line is provided in this Si02 film, and zinc is diffused from above to form a zinc diffusion portion 12 that reaches the active layer 4. Diffraction grating 14 provided on the surface of A〆zGal-zAs layer 5
Because of the optical feedback, the portion sandwiched between electrodes 10 and 1 constitutes a striped distributed feedback semiconductor laser 20' in which the excitation current flows through the window portion of the Si02 film. The striped portion in which zinc is diffused in the active layer 4 has a narrow effective forbidden band compared to the outside thereof, and has a high conductivity near the wavelength corresponding to this forbidden band. Therefore, the optimum oscillation wavelength is set to a longer wavelength side than that in the embodiment of FIG. Furthermore, stable oscillation with no abnormality in laser characteristics can be obtained due to the stable optical waveguide provided by the high yield ratio section. Fig. 3 is a conceptual diagram of the third embodiment of the present invention. be.

In this embodiment as well, the multilayer structures 2 to 7 are manufactured on the same machine as in the embodiment shown in FIG. There is no AsoGal-zAs layer 5 on it, and AsyG
The al-y rock layer 6 is grown directly. The Si02 film 8 has stripes 13 indicated by dotted lines on the back side of the surface electrode 10.
There is a window of this size at the position , and zinc is diffused from above the Si02 film 8 to the active layer 4 in this portion.
The part sandwiched between electrodes 10 and 1 is
The yGal-yAs layer 6 and the GaAs layer 7 constitute a semiconductor laser 22 which is excited by a current flowing concentratedly in the stripe portion formed by zinc diffusion and becomes capable of oscillation by optical feedback by the diffraction gratings 16 and 16. In this embodiment, the diffraction gratings 15 and 16 are not installed in the part of the active layer 4' which is sandwiched between the electrodes 10 and 1 and excited by the current, but are installed outside the excitation region at both ends thereof. Therefore, the semiconductor laser 22 is a distributed Bragg reflector laser.
Hectorlaser). In this embodiment as well, the grating spacing between the diffraction gratings 15 and 16 is set to the optimum grating spacing as in the embodiments of FIGS. 1 and 2. In this case, the effect of zinc diffusion is, in addition to the effect explained for the embodiment of FIG. The objective is to improve the efficiency of optical feedback of these diffraction gratings and to improve the oscillation efficiency of the semiconductor laser 22. The first of the above
In the embodiments 3 to 3, the distributed feedback type or distributed bragged reflector type semiconductor laser 20, 20' or 22 is the optical modulator 21'.
Although these are monolithically coupled, the latter may be a simple optical waveguide or may be another type of optical functional element.

That is, the present invention provides that in the active layer 4 of the distributed feedback type or distributed Bragg reflector type semiconductor laser 20, 20' or 22, a non-excited region exists inside or outside of these semiconductor lasers, and the output of these semiconductor lasers is In a composite semiconductor device or an optical integrated circuit including a distributed feedback type or distributed plug reflector type semiconductor laser 20' or 22 in which all or a portion of the laser beam must pass through these non-excited regions, As explained in detail above, according to the present invention, the phenomenon in which the forbidden band width of a semiconductor is reduced by free carriers or excited carriers is skillfully utilized to create a ``practical material with substantially less loss.'' A composite semiconductor device including a distributed feedback type or distributed Bragg reflector semiconductor laser can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS Wings 1 to 3 are conceptual diagrams of the first to third embodiments of the present invention, respectively.

In the figure, "1, 10, 1 river and electrode 4 are A do phantom Ga
l-xAs active layer, 12 is a zinc diffusion part, 13 is a stripe part in which zinc is diffused, and 14 is a diffraction grating for distributed feedback.
15 and 16 are diffraction gratings provided on the surface of the active layer 4; 20
, 20' is a distributed feedback semiconductor laser having a multilayer structure, 2
1 is an optical modulator, and 22 is a distributed Bragg reflector type semiconductor laser having a multilayer structure.

O diagram 2nd figure 3rd figure

Claims (1)

[Scope of Claims] 1. Consisting of a distributed feedback type or distributed bragg reflector type semiconductor laser and an optical functional element monolithically coupled to this semiconductor laser, the light of the distributed feedback type or distributed bragge reflector type semiconductor laser A composite semiconductor characterized in that the lattice spacing of the diffraction grating that performs feedback is set so as to perform effective optical feedback at a wavelength that is 50 Å or more longer than the wavelength corresponding to the forbidden band width when the active region of the laser is not excited. Device. 2. Consisting of a distributed feedback type or distributed bragg reflector type semiconductor laser and an optical functional element monolithically coupled to this semiconductor laser, a stripe in a part of the active region of the distributed feedback type or distributed bragg reflector type semiconductor laser 1. A composite semiconductor device characterized in that by adding an impurity to a shaped portion, the dielectric constant of the portion is increased and the effective forbidden band width of the portion is reduced. 3. The composite according to claim 2, wherein impurities are added to a striped portion of a part of the active region of a distributed feedback type or distributed Bragg reflector type semiconductor laser by zinc diffusion. Semiconductor equipment. 4. The lattice spacing of the diffraction grating for optical feedback of a distributed feedback type or distributed bragged reflector type semiconductor laser is 55% smaller than the wavelength corresponding to the forbidden band width when the active region of the laser is not excited.
4. The composite semiconductor device according to claim 2, wherein the composite semiconductor device is configured to perform effective optical feedback at wavelengths longer than 0 Å.